1. Field of the Invention
The present invention relates to a radiation-image processing device and method for performing gradation processing on a radiation image.
2. Description of the Related Art
A radiation image used for image diagnosis is acquired through imaging in a computed radiography (CR) system or imaging using a radiation detector (FPD), but a dynamic range of the acquired radiation image is very wide. When an entire dynamic range of the radiation image having a wide dynamic range is converted into an image, a contrast of the image is lowered and is not suitable for diagnosis. Therefore, in order to adjust the dynamic range of the radiation image and obtain an appropriate density and contrast of an image to be displayed, image processing such as gradation processing or frequency enhancement processing is performed. As a scheme of gradation processing, for example, a scheme of obtaining a histogram of pixel values of a radiation image, switching algorithms or parameters based on accessory information such as an imaged portion of the radiation image, an imaging direction, and an age of test substance, generating a portion (main histogram) corresponding to a main region in the histogram, and assigning a gradation curve so that a density range of a main histogram falls within a dynamic range of a display device has been proposed (for example, JP1994-61325B (JP-H06-61325B)). Further, a scheme of setting a region of interest (ROI) in a region for attention on the radiation image, switching algorithms or parameters based on the accessory information of the radiation image, and allocating a gradation curve so that a reference value of a pixel value within the ROI (for example, a maximum value or a median value) becomes a predetermined density has been proposed.
However, in the above-described scheme, since accessory information such as the imaged portion and the imaging direction of the radiation image and the age of the test substance is necessary, it is necessary for the accessory information of the radiation image to be input to the image processing device that performs image processing each time a radiation image is captured. Here, the accessory information is input by an operator who performs imaging selecting an imaging menu in the image processing device. For example, approximately 100 to 300 imaging menus are often used although the number of imaging menus varies depending on a facility. Accordingly, for the operator who performs imaging, work of selecting a desired imaging menu becomes very complicated, and mistyping easily occurs. Further, in order to perform optimum image processing, it is necessary to set optimum image processing parameters for each imaging menu, and an adjustment work for the setting also becomes very complicated. Meanwhile, by introducing a radiology information system (hereinafter referred to as an RIS), an imaging menu can be assigned to order information of the imaging, and thus, it is not necessary to perform work of inputting the imaging menu. However, in the case of a facility in which the RIS is not introduced, the operator needs to perform input of the imaging menu each time at the time of imaging.
In large hospitals, since there are image processing personnel, it is easy to make a modification when image quality of a radiation image cannot be satisfied. However, in small hospitals, since it is not easy to employ image processing personnel, modifying a radiation image to obtain image quality at the same level as obtained by image processing personnel becomes very difficult work. Thus, there is need for an image processing device capable of automatic conversion to an image suitable for diagnosis regardless of test substance or imaging conditions (a tube voltage, an amount of radiation, positioning, or the like).
In JP4844560B, a scheme of extracting a region of interest from a radiation image and converting a gradation of the radiation image so that a density of the extracted region becomes a desired density has been proposed. The scheme described in JP4844560B is a scheme of extracting a bone portion or a soft portion included in the radiation image as a region of interest, generating a weighted image by weighting the extracted region of interest, multiplying the weighted image by the radiation image to create a weighted histogram, evaluating the weighted histogram using a predetermined evaluation function, calculating a shift value of the histogram at which an evaluation value is maximized, and determining image processing conditions so that a predetermined processing result is obtained in a pixel value of the radiation image corresponding to a maximum value of the evaluation function used to obtain this shift value and, particularly, image processing conditions so that a pixel value corresponding to the shift value becomes a desired density. According to the scheme in JP4844560B, since the region of interest is extracted from an image without using accessory information of the image and the image processing conditions are determined for each image using the extracted region, it is possible to obtain a radiation image subjected to gradation processing without input of an imaging menu.
Incidentally, in diagnosis using a radiation image, it is important to recognize a change in the inside of a body by observing the image. For example, if inflammation is generated in a lung field, a portion thereof exhibits a low density (high luminance) in a radiation image. Further, when diagnosis of osteoporosis is performed, a portion thereof exhibits a high density (low luminance) in the radiation image. Therefore, in the diagnosis using a radiation image, it is important to perform image processing to reflect a change in a composition in a body in a change in the density of the image.
However, in the scheme described in JP4844560B, since a bone portion or a soft portion considered to be important is converted to have a predetermined density, when a tissue change such as inflammation occurs in a tissue of the bone portion or the soft portion considered to be important, the tissue change cannot be reflected in a change in the density of the radiation image.
Therefore, a scheme of performing gradation processing using a density of a region of which the density does not change even when the composition changes physically in the radiation image has been proposed (JP2000-079110A or the like). In the scheme described in JP2000-079110A, a thoracic region and a shoulder (clavicle) region are extracted as regions always maintaining the same density in a radiation image of a chest portion, and lesions of other regions are observed based on a representative value of this region.